The present invention relates to the field of digital broadcasting, and more particularly the insertion of digital video streams into other digital video streams.
Digital television involves the generation and storage of enormous quantities of data. Digital compression techniques can be applied to this data to drastically reduce the volumes of data required for transmission and storage. One compression technique common in the field is MPEG-2. MPEG-2 compresses video data by removing or reducing redundancy inherent in many types of image or video sequences. MPEG-2 makes use of three different types of frame which enable redundancy to be minimised. The three types of frames used are known as I frames, B frames and P frames.
I frames contain information which allows a complete frame to be reconstructed from only the data contained within the I frame. P frames use a single previously reconstructed frame as the basis for temporal prediction calculations. P frames base their predictions on the nearest I or P frame, and this is known as forward prediction. B frames use bi-directionally interpolated motion prediction to allow a decoder to rebuild a frame that is located between two reconstructed display frames. B frames use both past frames and future frames to make their predictions and require more than two frames of video storage. MPEG-2 video streams are made of a sequence of I, P and B frames which describe the video sequence.
The decompression and display of MPEG-2 compressed video streams may therefore rely on both past and future frames. Given the nature of compressed video streams, certain functions such as editing and insertion of bit-streams becomes problematic. If a compressed video sequence is cut at any point in time it is likely the frame immediately prior to the cut may well be dependent on information contained in subsequent frames to complete the decoding process. Also, if a video sequence is inserted into a video stream it is also likely that the first frames of the video sequence are based on previous frames which no longer exist.
If any frame required by the decoding process, such as a previous or future frame, is missing, this will lead to temporary breakdown of the decoding process until the next I frame is received. This will result in a temporary reduction of quality of the decoded image.
The problem could be avoided if frame accurate insertion is not required, however this is not a suitable solution for the broadcaster.
A problem therefore arises where frame accurate editing and insertion of compressed video bit-streams is required.
Accordingly, one object of the present invention is to provide a method and apparatus to enable frame accurate editing and insertion of compressed video streams.
According to one aspect of the present invention there is provided a method of processing a compressed digital bit-stream including a sequence of temporally referenced frames, at least some of which are coded in dependence on information in preceding or succeeding frames, to allow the bit-stream to be inserted into another such digital bit-stream, the method comprising the steps of: identifying the presence of one or more frames at a given insertion point which are coded in dependence upon one or more frames beyond the insertion point; and modifying the sequence so as to remove any such dependency and maintain continuity of the temporal references.
According to a second aspect of the present invention there is provided apparatus for processing a compressed digital bit-stream including a sequence of temporally referenced frames, at least some of which are coded in dependence on information in preceding or succeeding frames, to allow the bit-stream to be inserted into another such digital bit-stream, the apparatus comprising the steps of: a detector for identifying the presence of one or more frames at a given insertion point which are coded in dependence upon one or more frames beyond the insertion point; and a processor for modifying the sequence so as to remove any such dependency and maintain continuity of the temporal referencing.